Multimedia is being touted as telecommunications of the future. Telecommunication networks are required to handle a variety of traffic such as voice, video, data etc.; in other words, the bandwidth must be shared by many different classes of traffic. However, different classes of traffic call for different qualities of service from the network, with respect to, for example, speed of transmission, probability of transmission error, network delay etc. As for the network delay, interactive data, voice and video traffic are delay sensitive and sensitivity is characterized by full duplex operation where users expect timely service. Considering the consequences of slow turn-around on customer satisfaction, slow interactive operations are deemed to be inadequate. Non-delay-sensitive traffic includes bulk down-load, non-real time video. This traffic can sustain relatively large delays because the end users don't see the delay. Typically, these services are simplex and delay is transparent to the user.
ATM and Fast Packet switching architectures are being proposed for integrated switching applications. These broadband technologies are characterized by cell switched data. The main advantage of cell switching is that it minimizes switching delays caused by voice/video/data interactions and it is suitable for hardware accelerated routing. Cell switching provides a predictable and controllable delay. Long data packets do not adversely interfere with delay sensitive voice packets.
Cell switching provides a connection oriented routing algorithm which prevents packet mis-ordering. By establishing an explicit path, cells are guaranteed to arrive in sequence. Under normal circumstances, all cells associated with a connection follow an explicit path.
Cell switching effectively solves the emission delay and sequencing problem for connection oriented services, however, it is not well suited to connectionless data services. Packet switching connectionless data services over a broadband cell switch results in out of sequence cells due to multi-path delay variance. The re-assembly of out of sequence cells into packets poses a significant processing requirement on the network end stations. Furthermore, the transport of large (10-20 Kbyte) data packets requires processor intensive packet fragmentation into cells and re-assembly of cells into packets.
In the past, bandwidth sharing schemes have been proposed for integrated packet switching over a switching network. One scheme for an integrated voice and data network is described in U.S. Pat. No. 4,914,650, issued on Apr. 3, 1990 to Sriram. This network has a multiplexer arranged with a voice queue for storing received voice packets and a data queue for storing received data packets. Voice packets are transmitted for a predetermined interval T1 and data packets are transmitted for a predetermined interval T2. Predetermined intervals T1 and T2 may be of different duration. The multiplexer may be additionally arranged with a separate signaling queue for storing received signaling messages. If a signaling message is moved into the separate signaling queue during either interval T1 or T2, that interval is suspended and transmission of voice or data packets is interrupted from the end of a packet until the signaling message is transmitted. Then transmission of the interrupted voice or data packets is resumed for the remainder of the suspended interval T1 or T2.
The Sriram patent also states that alternatively a multiplexer may be arranged with a separate signaling queue for storing received signaling messages at any time. Signaling message transmission intervals are reserved either between intervals T1 and T2, between intervals T2 and T1, or both. These signaling intervals are of a flexible duration that is sufficiently long for transmitting all of the signaling messages waiting to be transmitted. The multiplexer allocates a certain minimum bandwidth for voice and data traffic. This protects each type of traffic from congestion caused by the other type. Concurrently, the multiplexer also allocates to each type of traffic any spare bandwidth momentarily available because it is not being utilized by the other type of traffic. Signaling messages are serviced with very low delay and zero packet loss.
In U.S. Pat. No. 4,707,831, issued Nov. 17, 1987 to Weir et al, an electrical intelligence transmission system is described. In the system, both speech and data are transmitted as packets over virtual connections set up in a fully digital network, the speech and data packets being transmitted at the bit rate of the transmission medium in which speech packets have priority over data packets. If a speech packet is detected during the transmission of a data packet, the transmission of that data packet is interrupted to allow the speech packet to be conveyed over the transmission medium, and when the medium again becomes free of speech, the non-transmitted part of the interrupted data packet is transmitted as a separate packet.
In the system of Weir et al, both speech and data are transmitted as packets over virtual connections set up in a fully digital network. A virtual connection is set up from a calling party to a called party at a call setup. Each packet contains source address, destination address, routing information of the switching network etc., in addition to payload and other network management information. Each packet stays intact while being transported through the network except for network management information and routing information. Each packet is transmitted through the network to the destination along the route specified in the routing information. Therefore, if the customer desires to send voice to a destination while data packets are being transmitted to a different destination over the same loop, the voice packet must interrupt the transmission of data packets because it requires minimum delay. The patent achieves this by adding a tail to the interrupted data packet and storing the remaining un-transmitted part of the data packet in a buffer. Like a data packet, a voice packet also contains addresses, routing information etc., but is 64 bytes long instead of 1024 bytes for a data packet. While the data packet is being interrupted, the voice packet must seize a channel in a synchronous TDM slot and is transmitted immediately when one is found. When the loop becomes available, the remaining part of the data in the buffer is sent as a separate packet containing addresses, routing information etc. In the patent, transmission is byte oriented and requires byte overheads because of channelized (timeslotted) design. As described above, packets are transported through a virtual connection from end to end. Therefore each packet must contain addresses, routing information, etc. The patent does not support multiple interruptions of a data packet. This means that the patent cannot guarantee delay of high priority packets. For example, if a link has a 16K packet to emit and is stopped after only 1 data byte is emitted in order to insert voice, 64 bytes of voice are now emitted and the remaining 15,999 bytes of data must then be played out, which will also delay any future voice packets. Because the patent limits the data packet to 1K and voice to 64 bytes, this may not be a problem for a loop circuit. However, in a high density network, this technique cannot be used without severe performance restrictions.